1. Field of the Invention
This invention relates to a displacement and a load measuring apparatus of a rotating member. The displacement and a load measuring apparatus of a rotating member is utilized for ensuring a stable operation of a vehicle by rotatably supporting, for example, a wheel of a vehicle (automobile) with respect to a suspension and measuring a magnitude of a load applied to the wheel. Further, the displacement measuring apparatus and the load measuring apparatus of the rotating member are utilized for pertinently adjusting a speed of feeding a tool or the like by being integrated to a rolling bearing unit for supporting main spindles of various machine tools and measuring a load applied to the main spindle, or a displacement by thermal expansion or the like.
2. Description of the Related Art
For example, a rolling bearing unit is used for rotatably supporting a wheel of a vehicle with respect to a suspension. Further, in order to ensure a running stability of a vehicle, a running state stabilizing apparatus of a vehicle such as an antilock braking system (ABS) and a traction control system (TCS) is widely used. According to the running state stabilizing apparatus of ABS, TCS or the like, although a running state of a vehicle in braking or acceleration can be stabilized, in order to ensure the stability even under a severer condition, it is necessary to control a brake or an engine by inputting a number of information influencing on the running stability of the vehicle.
That is, in a case of the running state stabilizing apparatus of the background art of ABS, TCS or the like, there is carried out a so-called feedback control of controlling a brake or an engine by detecting a slip between a tire and a road surface, and therefore, the control of the brake or the engine is delayed even the delay is momentary. In other words, the slip between the tire and the road surface, or so-called pulling to one side of the brake, in which forces of braking left and right wheels differ from each other extremely, cannot be prevented by a so-called feed forward control in order to promote a function under a severe condition. Further, in a truck or the like, it cannot also be prevented that a running stability is failed based on a failure in a loading state.
In order to deal with such a problem, for carrying out the feedforward control or the like, it is conceivable to integrate a load measuring apparatus for measuring either or both of a radial load and an axial load applied to a wheel into a rolling bearing unit for supporting the wheel with respect to a suspension. There are known rolling bearing units for supporting a wheel attached with a load measuring apparatus which can be used in such a case, in a background art as described in Patent Documents 1 through 4.
In Patent Document 1, there is described a rolling bearing unit attached with a load measuring apparatus capable of measuring a radial load. In a case of a first example of the background art structure, by measuring a displacement with regard to a diameter direction between an outer ring which is not rotated and a hub which is rotated on an inner diameter of the outer ring by a noncontact type displacement sensor, a radial load applied between the outer ring and the hub is calculated. The calculated radial load is utilized for properly controlling ABS as well as informing a failure in a loading state to a driver.
Further, Patent Document 2 describes a structure of measuring an axial load applied to a rolling bearing unit. In a case of a second example of a background art structure described in Patent Document 2, at a plurality of portions of an inner side face of a fixed side flange provided at an outer peripheral surface of an outer ring, load sensors are attached respectively to portions surrounding screw holes for screwing bolts for coupling the fixed side flange to a knuckle. The respective load sensors are squeezed between an outer side face of the knuckle and the inner side face of the fixed side flange. In a case of a load measuring apparatus of a rolling bearing unit of the second example of the background art, an axial load applied between the wheel and the knuckle is measured by the respective load sensors.
Further, Patent Document 3 describes a structure in which by displacement sensor units supported at positions of 4 portions in a circumferential direction of an outer ring and a detected ring having a section in an L-like shape outwardly fitted to fix to a hub, displacements in a radial direction and a thrust direction of the hub relative to the outer ring at the positions of 4 portions are detected, and based on detected values of respective portions, a direction of a load applied to the hub and a magnitude thereof are calculated.
Further, Patent Document 4 describes a method of providing a strain gage for detecting a dynamic strain at an outer ring corresponding member a rigidity of which is partially reduced, calculating a revolving speed of a rolling element from a frequency of passing the rolling element detected by the strain gage, and measuring an axial load applied to the rolling bearing from the revolving speed.
In a case of the first example of the background art structure described in Patent Document 1, the load applied to the rolling bearing unit is measured by measuring the displacement with regard to the diameter direction of the outer ring and the hub by the displacement sensor. However, an amount of the displacement with regard to the diameter direction is small, and therefore, in order to accurately calculate the load, as the displacement sensor, a highly accurate one needs to be used. The highly accurate noncontact type sensor is expensive, and therefore, it is unavoidable to increase cost of a total of the rolling bearing unit attached with the load measuring apparatus.
Further, in a case of the second example of the background art structure described in Patent Document 2, it is necessary to provide the load sensors by a number the same as that of the bolts for supporting to fix the outer ring by the knuckle. Therefore, not only the load sensor itself is expensive, but also it is unavoidable to considerably increase cost of a total of the load measuring apparatus of the rolling bearing unit. Further, according to the structure described in Patent Document 3, the sensor are installed at positions of 4 portions in a peripheral direction of the outer ring, and therefore, cost is increased more than that of the structure described in Patent Document 1. Further, according to the method described in Patent Document 4, it is necessary to partially reduce the rigidity of the outer ring corresponding member, and there is a possibility of making a durability of the outer ring corresponding member difficult to ensure.
Further, in the structure and the method described in any one of Patent Documents 1 through 4, an exclusive mechanism is provided for measuring the load applied to the rolling bearing unit. Therefore, it is unavoidable to increase the cost as well as a weight thereof.
Further, as a technology related to the invention, Patent Document 5 describes a structure in which by using an encoder alternately arranged with N poles and S poles at a detected face, a swing of the center of an inner ring supporting the encoder is detected. However, in Patent Document 5, there is not described a technology calculating a load applied to a rolling bearing unit by utilizing the encoder even when a description suggesting such a technology is included.
In contrast thereto, the inventors have invented a structure in which based on a pattern of a change in an output signal of a sensor which is changed in accordance with rotation of an encoder mounted to a hub constituting a rolling bearing unit, a direction and a magnitude of a load applied to the rolling bearing unit are calculated (Japanese Patent Application No. 2005-147642). In a case of the rolling bearing unit attached with a load measuring apparatus according to the previous invention, the encoder is concentrically supported by and fixed to a portion of a rotatory bearing ring such as the hub, and a detecting portion of the sensor supported by a portion which is not rotated is made to be opposed to a detected face of the encoder. Further, a width direction of the detected face is made to coincide with a direction of operating a load to be calculated. Further, a property of the detected face is alternately changed in a circumferential direction, and a pitch or a phase of the property changed in the circumferential direction is continuously changed in the width direction of the detected face. When the load is applied to the rotatory bearing ring, a position in the width direction of the detected face of the encoder opposed to the detecting portion of the sensor is changed and the pattern of varying the output signal of the sensor is changed. There is a correlative relationship between a degree of changing the pattern and the magnitude of the load, and therefore, by observing the pattern, the magnitude of the load can be calculated.
However, for example, in a case of a rolling bearing unit for supporting a wheel, in order to ensure a running stability of an automobile, a rigidity thereof is considerably high. Therefore, an amount of a relative displacement between a stationary bearing ring and a rotatory bearing ring generated based on a load is small even when the load is a radial load or an axial load. For example, when the rolling bearing unit for supporting the wheel is operated with an axial load of about 10 kN, an amount (length) of relatively displacing the stationary bearing ring and the rotatory bearing ring in an axial direction is only about several tens μm through several hundreds μm. Further, when a radial load of about 10 kN is applied, an amount of relatively displacing the stationary bearing ring and the rotatory bearing ring in a diameter direction is only about several tens μm.
As is apparent from the above-described explanation, in order to calculate the axial load by an accuracy which can be reduced to practice in order to ensure a running stability of the automobile, the displacement in the axial direction of about several μm through several tens μm needs to be calculated based on a detecting signal of the sensor (in accordance with a rigidity or the like of the rolling bearing unit for supporting the wheel). Further, with regard to the radial load, the displacement in the diameter direction of about several μm or smaller needs to calculate based on the detecting signal of the sensor (in accordance with the rigidity or the like of the rolling bearing unit for supporting the wheel). In order to calculate the displacement having the small amount, a boundary (a boundary between a concave portion and a convex portion formed at the detected face, or a boundary between an N pole and an S pole magnetized on the detected face) at which the property of the detected face of the encoder is changed needs to be fabricated accurately (a position or an angle of inclination at the boundary line needs to be fabricated in accordance with a design value). In contrast thereto, there is a limit in a working accuracy or a magnetizing accuracy, it is necessary to take into consideration to some degree that the pattern of varying the detected signal of the sensor is changed based on a dimensional error of the boundary line.
Further, even if the accuracy of the boundary line by which the property of the detected face of the encoder is changed can be satisfied, by an integration error in integrating the encoder to the rotatory bearing ring, in accordance with rotation of the rotatory bearing ring, the detected face is apparently displaced (vibrated in accordance with rotation) regardless of the load. When the load operated between the stationary bearing ring and the rotatory bearing ring constituting the rolling bearing unit is calculated by the structure of the previous invention, so far as the output signal of the sensor is not corrected, it is important that a geometrical center axis and a rotation axis center of the detected face of the encoder coincide with each other. When two center axes do not coincide with each other, that is, the two center axes are shifted from each other in the diameter direction, or inclined to each other, regardless of the load, the position in the width direction of the detected face to which the detected portion of the sensor is opposed is shifted.
For example, in a case in which the load to be detected is the radial load, the detected face of the encoder becomes a side face in the axial direction, when positions in the diameter direction of a center axis of the side face in the axial direction and the rotation center axis of the rotatory bearing ring are shifted from each other, the detected face carries out a whirling movement of a first order of rotation in accordance with rotation of the rotatory bearing ring. Further, when the load to be detected is the axial load, there is frequently a case in which the detected face of the encoder is constituted by a peripheral surface thereof, when a center axis of the peripheral surface and the rotation center axis of the rotatory bearing ring are inclined to each other, the detected face carries out a movement of displacing in the axial direction of a first order of rotation in accordance with rotation of the rotatory bearing ring. In either case, a portion of the detected face opposed to detected portion of the sensor is shifted in the width direction of the detected face. As a result, even when the load is not varied, a pattern of varying the output signal of the sensor is changed, and a measurement accuracy of the load is deteriorated.
Patent Document 1: JP-A-2001-21577
Patent Document 2: JP-A-3-209016
Patent Document 3: JP-A-2004-3918
Patent Document 4: JP-B-62-3365
Patent Document 5: JP-A-2004-77159